shapeshifting is hard. to prevent shifting onto the wrong form as one's mind inevitably wanders in the process, it often takes a degree of focus and discipline. One technique that many have found useful is training themselves to only shift in response to a form of self produced stimulus - such as a snap of the fingers - so as to make sure they have the form they're changing into fixed firmly in mind.
of course, this also means that if you snap your fingers around your shapeshifting friends, well... Just don't do it around a dragon in an enclosed space.